There has recently been an increased interest in the use of ambient light detectors, e.g., for use as energy saving light sensors for displays, for controlling backlighting in portable devices such as cell phones and laptop computers, and for various other types of light level measurement and management. Ambient light detectors are used to reduce overall display-system power consumption and to increase Liquid Crystal Display (LCD) lifespan by detecting bright and dim ambient light conditions as a means of controlling display and/or keypad backlighting.
Without ambient light detectors, LCD display backlighting control is typically done manually whereby users will increase the intensity of the LCD as the ambient environment becomes brighter. With the use of ambient light detectors, users adjust the LCD brightness to their preference, and as the ambient environment changes, the display brightness adjusts to make the display appear uniform at the same perceived level; this results in battery life being extended, user strain being reduced, and LCD lifespan being extended. Similarly, without ambient light detectors, control of the keypad backlight is very much dependent on the user and software. For example, keypad backlight can be turned on for 10 second by a trigger which can be triggered by pressing the keypad, or a timer. With the use of ambient light detectors, keypad backlighting can be turned on only when the ambient environment is dim, which will result in longer battery life.
In order to achieve better ambient light sensing, ambient light detectors should have a spectral response close to the human eye response and have excellent Infrared (IR) noise suppression. Currently, most companies use proprietary processes or special optical packages to build ambient light detectors with human-eye response and IR rejection.
For various reasons, there is an interest implementing ambient light detectors using complementary-metal-oxide semiconductor (CMOS) technology. First, CMOS circuitry is generally less expensive than other technologies, such as Gallium Arsenide or bipolar silicon technologies. Further, CMOS circuitry generally dissipates less power than other technologies. Additionally, CMOS photodetectors can be formed on the same substrate as other low power CMOS devices, such as metal-oxide semiconductor field effect transistors (MOSFETs).
FIG. 1A shows a cross section of an exemplary conventional CMOS light sensor 102, available in a conventional CMOS image sensor process. The light sensor 102 is essentially a single CMOS photodiode, also referred to as a CMOS photodetector, or a CMOS photocell. The light sensor 102 includes an N+ region 104, which is heavily doped, a N− region 106 which is lightly doped and a P− region 108 (which can be a P− epitaxial layer), which is lightly doped. All of the above can be formed on a P+ or P++ substrate (not shown), which is heavily doped.
Still referring to FIG. 1A, the N− region 106 and P− region 108 form a PN junction, and more specifically, a N31 /P− junction. This PN junction is reversed biased, e.g., using a voltage source (not shown), which causes a depletion region (not shown) around the PN junction. When light 112 is incident on the CMOS photodetector 102, electron-hole pairs are produced in and near the diode depletion region. Electrons are immediately pulled toward N− region 106, while holes get pushed down toward P− region 108. These electrons (also referred to as carriers) are captured in N+ region 104 and produce a measurable photocurrent, which can be detected, e.g., using a current detector (not shown). This photocurrent is indicative of the intensity of the light 112, thereby enabling the photodetector to be used as a light sensor. It is noted that FIG. 1A and the remaining FIGS. that illustrate light sensors are not drawn to scale.
A problem with such a conventional CMOS photodetector is that it detects both visible light and non-visible light, such as Infrared (IR) light. This can be appreciated from the graph in FIG. 1B, which illustrates an exemplary spectral response 120 of a human eye (as defined by International Commision on Illumination (CIE)), and a spectral response 122 of a conventional CMOS photodetector (e.g., 102) when housed in a low-cost transparent optical package. As can be appreciated from response curve 120, the human eye does not detect IR light, which starts at about 750 nm. In contrast, a conventional CMOS photodetector detects a significant amount of IR light, as can be appreciated from response curve 122. Thus, the spectral response 122 of a conventional photodetector significantly differs from the spectral response 120 of a human eye. This is especially true when the light 112 is produced by an incandescent light, which produces large amounts of IR light. Accordingly, the use of conventional CMOS photodetectors for adjusting backlighting, or the like, will provide for significantly less than optimal adjustments.
There is a desire to provide CMOS light detectors that have a spectral response closer to that of a human eye. Such light detectors can be used, e.g., for appropriately adjusting the backlighting of displays, or the like.